This invention relates to a method for separating bitumen oil from tar sands and the like.
The current industry practice for extracting bitumen from tar sands and the like is the hot water process, utilizing aggressive thermal and mechanical action to liberate and separate the bitumen. The hot water process is typically a three-step process. Step one involves conditioning the oil sands by vigorously mixing it with hot water at about 95 degrees Celsius and steam in a conditioning vessel to completely disintegrate the oil sands. Step two is the gravity separation of the sand and rock from the slurry, allowing the bitumen to float to the top where it is concentrated and removed as a bitumen froth. Step three is treatment of the remainder slurry, referred to as the middlings, using froth floatation techniques to recover bitumen that did not float during step two. To assist in the recovery of bitumen during step one, sodium hydroxide, referred to as caustic, is added to the slurry in order to maintain the pH balance of the slurry slightly basic, in the range of 8.0 to 8.5. This has the effect of dispersing the clay, to reduce the viscosity of the slurry, thereby reducing the particle size of the clay minerals.
A problem related to the industry practice is that the addition of caustic, coupled with the vigorous and complete physical dispersal of the fines, produces a middlings stream that may contain large amounts of well dispersed fines held in suspension. The recovery of bitumen from these middlings stream increases with the increase in the fines concentration over time. In addition, the middling stream that remains following step three, referred to as the scavenging step, poses a huge disposal problem. Current practice for the disposal of the resultant sludge involves the pumping of the sludge into large tailings ponds. This practice poses serious environmental risks.
The industry practice for the extraction of bitumen from oil sands has been to maximize the recovery of bitumen while minimizing the production of sludge, which require treatment and disposal. The industry practice typically provides for a bitumen recovery of between about 80% and 95% of the total amount of bitumen contained in the oil sands. Lower bitumen recoveries are experienced with oil sands of high fine material and low bitumen contents. To increase bitumen recovery, methods have arisen to reheat and recycle water recovered during the solids de-watering phase to re-expose the suspension of dispersed fine material to the conditioning bath, whereby the dispersed fine material may undergo further froth floatation treatment for bitumen recovery.